Solid state (semiconductor) image sensors are increasingly being used in consumer electronic devices such as mobile phones and various digital cameras. Image sensors convert optical radiation into electronic signals and are currently mostly implemented by digital charge-coupled device (CCD) devices or complementary metal-oxide-semiconductor (CMOS) devices. FIGS. 1A and 1B show a conventional front side illuminated (FSI) pixel. As shown in FIG. 1A, a metal layer 11 enables the transfer of electrical signal from the photodiode 10, and a nitride layer 12 acts as a passivation. An oxide layer 13 supports the metal layer and acts as a transparent homogenous medium between the photodiode and the nitride later. Additionally, a microlens layer 15 is placed on top of a filter layer 14 to focus the collimated light falling on the pixel 1, onto the photodiode 10. For achieving high quantum efficiency (QE), the focal length of the microlens layer 15 is designed such that the focal point falls on the photodiode. This produces a high QE for the on-axis collimated light 16, for example, a collimated light with 0° chief ray angle (CRA).
For lenses with high CRA angles at off-axis regions 17, the microlens and filter combination 18 are shifted for the pixels around the edges of the image sensor, as shown in FIG. 1B. This microlens (& filter) shifting also achieves a high QE for the off-axis collimated light with high CRA. For lenses with low CRA angles in off-axis regions, the filter and microlens combination 18 may not be shifted.
FIG. 1C shows an image sensor 10 that comprises of many rows and columns of the image pixel 1. As described above, the pixels around the edges of the image sensor 10, may have their microlens and filter combination shifted to compensate for the shift of the collimated light due to the position of those pixels
When the image sensor is placed and integrated in a camera, the light rays falling on the pixels of the image sensor are not collimated anymore, because of focusing from the camera lens, as shown in FIGS. 2A and 2B. Since the microlenses of the pixels are designed for collimated light, they do not focus the non-collimated incoming light rays at the photodiode. This leads to light loss and QE degradation, which becomes more significant in lowlight situations.
Typically, in a low light situation, digital cameras use a “fast lens” with low f-number to improve low-light sensitivity of the camera. That is, a camera lens with a larger maximum aperture (a smaller minimum f-number) is called a fast lens, because it delivers more light intensity to the focal plane and therefore achieves the same light collection with a faster shutter speed. However, additional light collected by fast lenses have steep angles which are attenuated by the pixels since they are not directly focused on the photodiode layer of the pixel. This leads to a minimum low light sensitivity improvement when fast lenses are used with FSI pixels. As a result there is a need for improved pixel QE for low light cameras.